Pseudocapacitive layered birnessite sodium manganese dioxide for high-rate non-aqueous sodium ion capacitors†
Layered transition metal oxides are promising cathodes for sodium ion capacitors due to their high specific capacity. In this work, we present a layered birnessite sodium manganese dioxide (Na0.77MnO2·0.5H2O) supported by a two-dimensional conductive network (denoted as b-NMO/C) as a cathode for non-aqueous sodium ion capacitor (SIC). The interlayer crystal water and carbon networks promote the ion/electron transport kinetics and overcome the structural instability, leading to largely enhanced electrochemical performance. As a result, the as-synthesized b-NMO/C cathode delivers a capacity of 192 mA h g−1 at 0.25C and 43 mA h g−1 even at a high rate of 100C. The attained performance is compared favorably with those of state-of-the-art Mn-based cathodes for sodium ion storage. Furthermore, the assembled asymmetric SIC (b-NMO/C//graphite) exhibits the highest energy (91 W h kg−1 achieved at ∼84 W kg−1) and power (5816 W kg−1 achieved at ∼37 W h kg−1) densities within the voltage range of 0.5–3.8 V.